Question

Please could someone advise me on answering this question. I need to explain why when infrared radiation falls on fluorescent material no visble light is emitted. Is this something to do with infrared being at the lower energy end of the spectrum?
I would be very grateful for some help as I'm struggling with it.

Yes, it has to do with the fact that infrared radiation has a lower frequency than ordinary light.

Light consists of photons which have an ergy of h f, where h is Planck's constant and f the frequency. Atoms and molecules can exist in different energy states. If a photon has an energy equal to the energy difference of two energy states, then it can be absorbed by the molecule. After some time, the molecule will fall back to the ground state and emit a photon.

The molecule can make a transition back to the ground state via intermediary states. Tghis means that the emitted photons will have lower energy and thus lower frequencies than the original photon.

This means that if infrared radiation is used to cause fluorescence, then the emiited radiation will be in the infrared.

Thank you kind sir. TYhis is a great help.